Chemical mechanical polishing method and apparatus

ABSTRACT

A method for removing material from the surface of a semiconductor wafer with a chemical mechanical polishing process is described. The method uses a polishing pad on which a line-pattern of grooves is formed. The pattern comprises orderly spaced grooved-area and area without grooves. The method combines information of the surface topography of the wafer, the nature of the material to be removed, and the available groove pattern on the surface of the polishing pad to generate a process recipe in which the resident time of portions of the semiconductor wafer spends at the grooved and un-grooved areas of the polishing pad during the chemical mechanical polishing process is pre-determined.

BACKGROUND OF THE INVENTION

This invention relates to semiconductor processing, particularly to achemical mechanical polishing method and apparatus with which to achievesuperior global and local wafer planarity control.

Modem integrated-circuit technology is capable of packing a large numberof circuit-components at near the surface of a semiconductor wafer byscaling down the feature size of the circuit-components and connects thecomponents with a large number of metal lines imbedded in a matrix ofmultiple layers of metal and dielectric material. Beneath the siliconwafer surface, the circuit-elements are isolated from each other byregions of silicon dioxide in shallow trenches to prevent unwantedelectrical current passage between the circuit-elements.

Because of the downward scaling of the feature size and the increasingcomplexity of interconnecting scheme, the wafer process requires a highdegree of wafer surface planarization. Specifically, the wafer surfacemust be planarized locally as well as globally after the formation ofthe shallow trenches and at all interconnect levels. Currently thetechnique of chemical mechanical polishing is the only satisfactorytechnique with which the necessary degree of planarization can beachieved.

In a chemical mechanical polishing operation, a semiconductor wafer ismounted upside down on a wafer-carrier, and the carrier is presseddownward against a polishing pad, which is motion with respect to therotating wafer-carrier. Slurry comprised of silica or cerium oxideparticles, for example, suspended in alkaline to slightly acid solutiondrips onto the polishing pad that has flow channels machined totransport the slurry beneath the rotating wafer carrier where itpolishes the wafer surface.

The most popular type of chemical mechanical polisher is the rotarypolisher in which both a platen and the wafer-carrier rotate. Thecarrier holds the wafer face down, and applies a downward force againstthe surface of the pad. The pad is mounted on a rotating platen bywaterproof adhesive. The wafer-carrier rotates about the center point ofthe wafer and it oscillates along a radius of the platen such that theentire wafer surface contacts the polishing pad during the polishingoperation.

Some types of the polishing pads are made of materials that absorb theslurry; other types are made of materials that do not have the abilityto absorb the slurry.

The rate of removal of the target material is a function of the pressurethat the pad exerts on the wafer surface, the relative speed between thepad and the wafer surface, and the nature of the slurry.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 depicts a cross section of a semiconductor wafer.

FIG. 2 depicts a polishing pad with a flow-channel pattern of thepresent invention.

FIG. 3 depicts a cross section of another semiconductor wafer.

FIG. 4 depicts another polishing pad with a flow-channel pattern of thepresent invention.

FIG. 5 depicts a flowchart of a chemical mechanical polishing process ofthe present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The applicants recognize that with today's rotary chemical mechanicalpolisher, different portions of the wafer experience different relativespeed with respect to the polishing pad during the polishing operation.The applicants also recognize that different processes that deposit thetarget material on the wafer surface may effects different built-innon-planarity on the wafer surface. In certain instances, the targetmaterial coating on the wafer may be thicker at the center while inother instances the coating is thicker at the edges.

The applicants further recognize that the rate of removing the targetmaterial depends on the presence of the slurry at the point where thewafer contacts the polishing pad. With a polishing pad made of materialsuch as polyurethane, polycarbonate, nylon, acrylic polymer, orpolyester, the slurry is transported to the contact point by the flowchannels machined onto the polishing pad. Therefore, the rate of targetmaterial removal is a function of the dimension and density of the flowchannels—a wafer gliding over an area on the polishing pad that has moredensely placed flow channels will have more of the target materialremoved than over an area that has more sparsely placed flow channels.

Therefore, the applicants discovered that with a properly designedflow-channel pattern, desired planarity can be achieved even when thesurface topography of the target material as formed is highlynon-planar. The following exemplary embodiments are for the purpose ofdescribing this invention.

FIG. 1 depicts the cross-section view of a silicon wafer 10 with adeposited layer of silicon dioxide 20. The dioxide layer may be formedfollowing the formation of shallow trench isolation.

In FIG. 1, the silicon dioxide layer 20 is outwardly tapered—itsthickness at the center of the wafer is greater than at the edge of thewafer. In order to compensate for this outward-tapering, it isadvantageous to use a polishing pad that has a pattern of flow channelsas depicted in FIG. 2.

FIG. 2 depicts a polishing pad 100, which may be made of material suchas polyurethane. FIG. 2 also depicts two areas A and B of the polishingpad and the enlargements A′ and B′ of the two areas, and the flowchannels in the areas. Note that the spacing between the flow channelsin area A is ‘a’ and the spacing between the flow channels in area B is‘b’. To polish an outwardly tapered wafer, the more favorableflow-channel pattern for polishing pad should be such that ‘a’ is widerthan ‘b’—in some cases, area A may be free of any flow-channels. It isapplicants' observation that during the polishing operation, the waferand the wafer carrier oscillates along a line OP while rotating. As aresult, the edge of the wafer travels along the edge of an ellipse Ewhile the center portion of the wafer is confined in the interior of theellipse E. A flow channel pattern such as depicted in FIG. 2 willachieve a polishing rate lower at the edge of the wafer because of themore sparsely spaced flow channels at the location A.

FIG. 3 depicts the cross-section view of another silicon wafer 30 with alayer 40 coated on the top surface. In this case, the layer 40 isinwardly tapered so that it is thinner at the center than at the edge ofthe wafer. Layer 30 may be deposited on the silicon with a spin-onprocess. In order to compensate for inward-tapering, it is advantageousto use a polishing pad that has a pattern of flow channels as depictedin FIG. 4.

FIG. 4 depicts another polishing pad and two areas C and D on thepolishing pad. Also depicted in FIG. 4 are the enlargements C′ and D′ ofthe two areas and the flow channels in the areas. The spacing betweenthe flow channels in area C is ‘c’ and the spacing between theflow-channels in area D is ‘d’. To polish a inwardly-tapering wafer, themore favorable flow-channel pattern should be such that ‘d’ is widerthan ‘c’—in some cases D may be free of any flow-channels. A polishingpad having a flow-channel pattern as depicted in FIG. 4 removes thetarget material more slowly at the center of the wafer because of themore sparsely spaced flow-channels.

FIG. 5 is a flowchart for a chemical mechanical polishing process of thepresent invention. The process starts by providing a wafer having acoating of target material with a known surface topography to bepolished 200. This topography may be ascertained by scanning the topsurface of each wafer or by sample-scanning a representative wafer in awafer-lot. As explained earlier, the topography of the coating isgenerally a function of how the coating is formed and the nature of thecoating material. In case of a spin-on film such as a spin-on-glass, thefilm tends to be inwardly tapered as the spun-on material gets pushedoutwardly by the centrifugal force of spinning operation to the edge ofthe wafer. In case of electro-plating, the location of the electrode andthe current path in the wafer surface may cause the film to be outwardlytapered.

The next step is to select a polishing pad of a proper flow-channelpattern 300. As explained in the previous paragraphs, for a givenrelative polishing speed, an area of denser flow-channel usually removestarget material at a higher rate.

The next step is to provide a recipe for the CMP operation 400. Therecipe may include the rotational speed for the platen—polishing pad andthe rotational speed for the wafer-carrier, the frequency of the wafercarrier oscillating along a radius of the platen, the slurry feedingrate, and endpoint detection data or a predetermined polishing time.

The next step is the actual polishing 500 of the wafer according to therecipe 400. When the predetermined polishing time is reached or when theendpoint detection mechanism triggers, the CMP operation is terminated600.

The present invention may be applied to semiconductor wafer other thansilicon. For example, the CMP method is applicable on compoundsemiconductor material. It is applicable on SiGe material. The targetfilm may comprise other material such as silicon, tungsten or copper.The flow-channel may be of more than one width and one depth.

1. A method for removing material from a surface of a semiconductorwafer with a chemical mechanical polishing (CMP) process, comprising: a.providing a semiconductor wafer having a top surface and a substantiallyflat bottom surface, the top surface substantially parallel the bottomsurface; b. providing a CMP apparatus including a polishing pad, thepolishing pad having a polishing surface and a backing surface, thebacking surface attaching to a platen, the polishing surface having apatterned portion and a un-patterned portion, the patterned portionhaving regularly spaced flow channels formed therein, the un-patternedportion free of flow channels; c. providing a process recipe forpolishing a semiconductor wafer based on a topographic information ofthe top surface, the recipe including directing a portion of thesemiconductor wafer to reside in the channel-free portion of thepolishing pad during a portion of the process; e. commencing the CMPprocess based on the process recipe; and f. terminating the CMP processwhen a predetermined amount of material is removed from the top surfaceof the semiconductor wafer.
 2. The method of claim 1, in which the flowchannels in the patterned portion form concentric rings uniformly spacedradially.
 3. The method of claim 1, in which the flow channels in thepatterned portion form concentric rings spaced increasingly densely froma central portion of the polishing surface outwardly to a peripheralportion of the polishing surface.
 4. The method of claim 1, in which thepatterned portion having concentric rings spaced decreasingly denselyfrom a intermediate portion of the polishing surface towards a centralportion and a peripheral portion of the polishing surface.
 5. The methodof claim 1, in which the un-patterned portion of the polishing surfaceforms an arc.
 6. The method of claim 5, in which the polishing surfacecomprises a plurality of the circular bands.
 7. The method of claim 6,in which the concentration of the plurality of the circular bands ishigher in the middle portion of the polishing surface than in thecentral and the peripheral portions of the polishing surface.
 8. Themethod of claim 6, in which the concentration of the plurality of thecircular bands is lower in the middle portion of the polishing surfacethan in the central and the peripheral portions of the polishingsurface.
 9. The method of claim 1, in which the flow channel is onecontinuous spiral.
 10. The method of claim 9, in which the lines of thespiral are spaced more sparsely at the center of the polishing pad. 11.The method of claim 9, in which the lines of the spiral are spaced moresparsely at the midpoint between the center and the edge of thepolishing pad.